The present invention relates to an improved propulsion system for a vehicle, such as an aircraft, with the propulsion system comprising an engine having an integrated hydrocarbon propellant rocket system positioned with respect to the engine so as to take advantage of the aft-body contour as a high area ratio self-compensating nozzle.
Existing airframe-integrated hydrocarbon fueled ramjet/scramjet engine concepts are designed to operate at flight velocities beginning at about Mach 4 and provide vehicle acceleration and cruise capabilities to as high as Mach 8. To provide the initial acceleration or boost to Mach 4, a variety of propulsion systems have been proposed including external solid or liquid rockets as well as ejector ramjets also referred to as Rocket Based Combined Cycle (RBCC) engines.
External rockets, although capable of providing high thrust levels beneficial to rapid acceleration and minimizing fuel burn, lack integrated vehicle efficiency or effective specific impulse (thrust minus drag divided by propellant flow rate). This is due to the fact that the airbreathing flowpath is not performing optimally during the low Mach number trajectory phase with inherent large aft facing areas contributing to large vehicle base drag.
RBCC systems, such as that shown in FIG. 1, by contrast, rely on relatively small rockets 10 located at or near the throat 12 of a scramjet engine 14 to entrain and pump airflow through the scramjet flowpath 16. The scramjet engine 14 typically has an inlet 18, the engine throat 12, a cowl 20, and a nozzle 22. Each rocket 10 comprises an ejector rocket with a contoured nozzle 24 and a piloting injector 26. Depending on the stoichiometry of the rockets 10 and the efficiency of air entrainment, fuel may be added downstream of the rockets 10 to provide additional afterburning. Although base drag is reduced over external rocket integrations, the scramjet nozzle pressure ratio is typically too low for the nozzle to pressurize appreciably resulting in the requirement for other base or external burning means to reduce drag. Thrust levels are fairly constrained as the rocket exhaust nozzles must be small enough not to adversely affect the scramjet engine performance via excessive throat blockage, form, and base drag within the flowpath.